Photonic detector devices respond to received photons by creating an electric effect which can be quantified and hence provide information on the flux of the received photons. Focal plane arrays (FPAs) of detectors are used to obtain images of objects, whereby each detector provides a pixel in the image array. In the image, each pixel is provided with a unique address and a numeric value, which can further be used for manipulating the image for extracting information from the image.
A trade-off between imaging parameters is a compromise often imposed for obtaining desirable image parameters. Accordingly, an increase in the image frame rate at the expense of image resolution or image size is known in the art.
Multi-mode sensors have become an important component in various applications, such as surveillance systems and search-and-track systems, etc. To ensure accuracy and proper target acquisition, it has become increasing more useful to obtain as much information as possible about the region of interest being imaged, such as, for example, detecting and tracking a potential target.
One common example of a multi-mode imaging device is a missile seeker that combines mid-wave infra-red imaging capability and semi-active laser detection and tracking. Infra-red imaging allows for tracking based on emitted heat or expected heat signature of a target, while a semi-active laser (SAL) detector allows the detection of a target being painted with a laser spot. With all of these advantages, there are several downsides to a multi-mode detector device. Generally, such devices require two or more disparate sensor systems whose outputs are combined and analyzed to give a true multi-mode detection capability. This causes such systems to generally be costly and complicated, limiting their use in the field.
General Description
There is a need in the art for a novel pixel readout circuit for use with an imaging pixel array. This is associated with the following:
Modern electro-optical systems are designed towards more compact, low power, and low cost systems compared to traditional systems. Integration of several components or functionalities, such as thermal imager, laser designator, laser range finder (LRF), into one multi-function detector can serve this trend. LRF becomes an increasingly vital component to high precision targeting engagements for the imaging system user. The precise and accurate range-to-target information is an essential variable in the fire control solution of today's sophisticated weapons. Handheld military range-finders normally operate at ranges of 2 km and up to 5 km, and have about ±10 m range accuracy. The more powerful models of range-finders measure a distance of up to 25 km and are normally installed either on a tripod or directly on a vehicle or gun platform. In the latter case, the range-finder module is integrated with a thermal night vision and daytime observation system. Lasers are also used extensively as Light Detection And Ranging (LIDAR) for 3-D object recognition. With the recent advances of LIDAR technology, the accuracy potential of LIDAR data has significantly improved. Conventional LIDAR systems can provide a pulse repetition rate of up to 100 kHz, and range measurement accuracy of a few centimeters.
Modern electro-optical systems contain several components such as a thermal imager, laser designator, laser range finder, etc. The demand for compact systems with low power consumption and low cost can be addressed by incorporating some of the traditional system abilities into the IR detector.
The present invention provides a pixel readout integrated circuit (ROIC) to be integrated with an active matrix light detector (photodiode) of any type creating an advanced multi-function infrared detector with on-chip processing. The present invention also provides a new type of detector comprising a Readout Integrated Circuit (ROTC) with advanced on-chip signal processing. For example, the ROTC incorporating a high level of signal processing, may be a flip chip-bonded to a 640×512 InSb detector array of 15 μm pitch.
There is thus provided a pixel readout integrated circuit to be integrated with an active matrix light detector (photodiode) of any type. For example, the active matrix detector may be a focal plane array detector (pixel matrix) made of materials sensitive to IR imaging (for example InSb, InAsSb, InGaAs, CMT, etc.), mid-wave IR imaging or short wave IR imaging or long wave IR imaging. The pixel readout is performed in an integrated circuit on-chip. The readout circuit is configured and operable to receive the entire pixel matrix detection data and to process the detection data to provide a single-pixel detection of an event at the focal plane identified as existence of an input pulse signal. More specifically, the readout circuit is capable of simultaneously or almost simultaneously implementing two different modes, an imaging mode and an event detection mode per pixel, which is implemented by processing data of the same frame. In the single-pixel event detection mode, the readout circuit identifies a specific event as a positive change in photocurrent, which might be indicative of the existence of a laser pulse or weapon fire pulse being any electromagnetic radiation pulse converted to current by the detector element. Due to the fact that such fast event detection is carried out concurrently with acquiring an image of the same frame, the fast event is not only detected but its location in the frame being imaged from the region of interest can also be determined, and, moreover, at the pixel-size resolution.
Therefore, according to one broad aspect of the present invention, there is provided a pixel readout circuit for use with an imaging pixel array. The pixel readout circuit comprises: (a) an input channel for receiving an image signal corresponding to electrical output of a photosensitive element of the pixel; (b) an electronic circuit interconnected between the input channel and an output readout utility, the electronic circuit comprising: (i) a capacitive unit comprising at least one capacitor, controllably linked to the input channel for accumulating a charge corresponding to a received intensity generated by the pixel during a single frame period, and connected to the output readout utility for transmitting image data thereto, and (ii) a signal analyzer unit controllably linked to the input channel for receiving and analyzing at least a part of the image signal generated by the pixel during the single frame period, and connected to the output readout utility.
The analysis of at least a part of the image signal generated by the pixel during a single frame period comprises detection of temporal changes in the accumulated current during the single frame period (corresponding to the intensity of the received signal), and comparing the magnitude of change to a preset threshold level. If the magnitude of change is greater than a predetermined condition (threshold), the signal analyzer unit generates data indicative of the detected event and transmits the data to the output readout utility.
In this connection it should be noted that if the change in signal intensity (the photo-current derivative) is larger than a predefined threshold (predetermined condition), an event is detected. The exact values depend on application trade-offs such as False-Alarm-Rate or power.
In some embodiments, the electronic circuit is configured to selectively vary conversion gain of the capacitive unit to provide a selected integration time of charge accumulation by at least one capacitor during a single frame period, thereby selectively providing different image acquisition modes with respectively higher and lower signal to noise ratio of the pixel operation.
In some embodiments, the pixel readout circuit comprises a switching assembly connected to the input channel and configured for selectively directing at least a part of the image signal to one or more units of the electronic circuit.
In some embodiments, the signal analyzer unit comprises a comparator configured and operable to determine a change in the amount of accumulated charge corresponding to the received intensity of the image signal by measuring a voltage difference.
In some embodiments, the signal analyzer unit is configured and operable to determine a time profile of accumulation of the charge corresponding to the received intensity of the image signal during the frame, and generating data indicative of a distance to a location in the region of interest originating the detected event.
In some embodiments, the signal analyzer unit comprises a time counter circuit measuring the time profile and generating data indicative of the distance by measuring a time of flight to the detected event. In this connection, it should be noted that the time counter circuit is an electronic circuit located on-chip. It may be internal to the pixel circuit or external to the pixel circuit. The time counter circuit may comprise a voltage ramp, a switch and a readout capacitor.
According to another broad aspect of the invention, there is provided a pixel readout circuit for use with an imaging pixel array, where the readout circuit comprises: (a) an input channel for receiving an image signal corresponding to electrical output of a photosensitive element of the pixel; (b) a plurality of electronic units, each electronic unit being interconnected between the input channel and a common output readout utility, each electronic unit being configured and operable for carrying out a different imaging mode by applying a different processing to at least a part of the same image signal; (c) a switching assembly comprising a plurality of switches, the switching assembly being connected to the input channel and controllably operable for selectively linking the input channel to one or more of the electronic units to thereby selectively direct at least a part of the image signal to one or more of the electronic units; and (d) a control system connected to the switching assembly and configured and operable to selectively activate one or more of the switches to perform the link between the input channel and the one or more different electronic units.
In some embodiments, the detector of the present invention can be operated in either one of the following four different modes of operation. The first operation mode is thermal imaging which may have functionalities and performance of a Mid-Wave IR imaging (MWIR) detector. The second operation mode is a dual-function mode that includes both thermal imaging and information on Asynchronous Laser Pulse Detection (ALPD) for each pixel. The detection probability of a laser pulse is significantly increased by integrating a dedicated in-pixel circuit for identifying a fast-changing signal by its temporal profile. Since each pixel has internal processing to identify pulses of electromagnetic radiation, it is possible also to measure the elapsed time between a trigger and the detection of a pulse. This yields a third mode of operation in which the detector is synchronized to a pulse source (e.g. laser) and becomes a Two-dimensional Laser Range Finder (TLRF). The fourth operation mode is dedicated to Low Noise Imaging (LNIM) for example for the Short Wave Infrared (SWIR) band, where the IR radiation signal is low. It can be used in either passive or active imaging.
In some embodiments, the readout circuit is also configured and operable to selectively operate in one of the multiple different modes of operation (one at a time). The readout circuit receives the pixel output current and generates processed data according to the selected mode of operation. The readout circuit carries out multiple different data processing modes, providing different types of information about a region of interest from which an image stream is collected. To this end, the readout circuit has multiple (typically four) separate data processing channels, each having its own electronic circuit.
The readout circuit comprises a switching assembly (appropriate arrangement of switches) which is responsive to the photo-diode input and is controllably operable to selectively switch one or more of the electronic circuits. To this end, the switching assembly defines a different input circuit characterized by predetermined conversion gain corresponding to the operational mode to be performed by the selected electronic circuit. Generally, for any imaging mode, the readout circuit is typically characterized by a predefined conversion gain, i.e. conversion of the input of the readout circuit (charge corresponding to output of the photodiode) to the output of the readout circuit (voltage). Such conversion gain is typically defined by capacitance of the readout circuit.
In some embodiments, one of the electronic circuits comprises a capacitive unit comprising at least one capacitor, controllably linked to the input channel for accumulating a charge corresponding to at least a part of the image signal generated by the pixel during a single frame period, and connected to the output readout utility for transmitting image data thereto.
The integrated readout circuit of the present invention integrated within an imaging pixel array provides a multi-mode detection system detecting image data in multi-mode from a single set of optics and detection components. These different modes can functionally be defined as follows:
a) regular imaging being an image acquisition mode by a pixel matrix, where all the pixels are concurrently exposed to light from a region of interest and their electrical outputs are detected and further read out.
b) low noise imaging being an image acquisition mode which differs from regular imaging by significantly higher conversion gain and thus higher signal to noise ratio for relatively weak signals. This mode is implemented by using a much smaller integration capacitor. In other words, a ratio between the conversion gains of the regular imaging and low noise imaging modes is high, e.g, a few order of magnitude. Low noise imaging utilizes a very high charge to voltage conversion ratio for the entire pixel matrix detection data.
c) event detection and spatial location of the event in the frame (at a focal plane in a 2D space) by a single pixel and single frame period. This mode allows for detection of an occurrence of an event as well as for determining a location of the certain event in the region of interest. The event detection input circuit is configured and operable to detect the charge accumulation for any given pixel during a single frame period and identify sudden changes in the rate of accumulation. The input circuit therefore detects current changes in real-time. The event is detected by measuring a current change indicative of a radiance change (e.g. laser pulse, gunshot, etc.) during each frame. In some embodiments, the change in current is detected by measurement of the current derivative instead of integrating the current.
d) time scale event location (electromagnetic pulse) and registration of the time of the event. The event location mode is implemented by detecting a distance to the event (via the time of flight measurement) for a given orientation of the pixel matrix with respect to the region of interest. In order to correctly measure the distance, the time of flight measurement is specifically triggered by a trigger unit interconnected between the photodiode and a time counter circuit including the corresponding switch of the readout circuit. A signal from the photodiode, while being generated, concurrently actuates the trigger unit. This mode thus enables range finding based on “time of flight” measurements, utilizing a starting point set by the trigger unit. The trigger unit activates a time counter circuit configured and operable to measure distances.
In both modes (c) and (d), the DC component of the detected signal is removed by using a band pass filter. Accordingly, a variation in the AC component in the frequency band of interest is detected, thus increasing sensitivity of the event detection and location (in space and time).
In some embodiments of the present invention, the readout circuit is capable of performing any combination of modes (a) to (d) including all these modes.
In other embodiments of the present invention, the readout circuit is capable of performing a combination of modes (a) and (b), namely regular imaging mode and a low noise imaging mode. Each of these modes is implemented in a snapshot fashion, namely simultaneously by all the pixels in the detector pixel matrix. Regular imaging and low noise imaging may be implemented alternatively (one at a time) using significantly different conversion gains for these modes. Typically, the ratio between the gains of the regular imaging mode and of the low noise imaging mode differs by three orders of magnitude. Therefore, these different imaging modes are suitable for different applications and different spectral ranges. The readout circuit is thus capable of hybrid spectrum detection.
In another embodiment of the present invention, the readout circuit is capable of performing a combination of modes (b) and (d), namely using the output of a low-noise detection circuit as input to the range finding circuit calibration.
In some embodiments, one of the electronic circuits comprises a signal analyzer unit controllably linked to the input channel for receiving and analyzing at least a part of the image signal generated by the pixel during the single frame period, and connected to the output readout utility. Analyzing at least a part of the image signal generated by the pixel during the single frame period comprises determining a change in an amount of accumulated charge corresponding to the received at least part of the image signal, and upon detecting that the amount of the accumulated charge satisfies a predetermined condition, generating data indicative of a detected event and transmitting the data to the output readout utility.
In some embodiments, at least one of the electronic circuits is configured and operable to acquire an image signal in a snapshot manner.